1. Field of the Invention
The present invention relates to a method and an apparatus for measuring the high-frequency C-V characteristics of a metal-insulator-semiconductor (MIS) device and more particularly to a method and an apparatus for measuring that of a metal-oxide-semiconductor (MOS) device.
2. Description of the Related Art
In obtaining characteristic values of a MIS device, e.g., a MOS capacitor, fabricated from a semiconductor substrate wafer, such as the carrier concentration N.sub.sub in the semiconductor substrate, the flat band voltage V.sub.fb required for flattening the energy band at the surface of the semiconductor substrate, the threshold voltage V.sub.th, and the charge density Q.sub.ss in the oxide film near the interface between the oxide film and the semiconductor, a high-frequency C-V characteristics is conventionally measured for the relation between the gate voltage and series connected capacitance (hereinafter referred to as MOS capacitance) of a gate oxide and the depletion layer near the semiconductor surface.
In the measurement of the high-frequency C-V characteristics, a voltage response of the MOS capacitance is measured when a voltage is applied to a gate electrode of the MOS device and the sweep direction of the applied voltage is turned from the accumulation to the inversion or vice versa. For example, when the voltage response of the MOS capacitance is measured so that the MOS device is exposed to light from incandescent lamp or fluorescent lamp, it is incorrectly measured.
FIG. 6 shows an actual high-frequency C-V characteristics of the MOS capacitor in solid line when the surface of the MOS capacitor is exposed to light from fluorescent lamp and a theoretical high-frequency C-V characteristics of the MOS capacitor in dotted line (see THE BELL SYSTEM TECHNICAL JOURNAL, September 1966, pp. 1097-1122). As shown in FIG. 6, in the range of inversion region, the MOS capacitance should be in equilibrium but actually increases, so that the actual curve is out of accordance with the theoretical curve. This is the cause that a light energy from fluorescent lamp induces excess minority carriers at the semiconductor surface. Thus, the MOS capacitance cannot be accurately measured under a condition of exposing the MOS capacitor to light from fluorescent or incandescent lamp.
Therefore, a prior-art apparatus of measuring the high-frequency C-V characteristics measures the high-frequency C-V characteristics in a shading and radiation-shielding box i.e. shielding-box. However, the measurement of the high-frequency C-V characteristics in the shielding-box has a drawback described below.
FIG. 7 shows in solid line an actual high-frequency C-V characteristics measured using the shielding box and a theoretical high-frequency C-V characteristics in dotted line. The MOS capacitance should be in equilibrium, i.e., constant. However, it actually decreases in inversion region and is entirely out of accordance with the theoretical curve, as shown in FIG. 7. Since the generation of minority carriers scarcely follow the sweep speed of the applied voltage, the depletion layer width increases with the applied voltage so that the equilibrium is not arrived. Therefore, the prior-art apparatus of measuring the high-frequency C-V characteristics cannot measure an exact MOS capacitance.
This drawback in the high-frequency C-V characteristics measurement is due to the following causes. In case of p-type silicon substrate, when the gate voltage is swept from the accumulation to the inversion, an electric field produced by a negative voltage applied to the gate electrode accumulates majority carriers at the semiconductor surface. This state is the accumulation in which the MOS capacitance consists only of the oxide capacitance and indicates the maximum value. Since the applied voltage to the gate is subsequently gradually increased in magnitude, the majority carriers are depleted at the semiconductor surface. This state is the depletion in which the MOS capacitance consists of the oxide capacitance and the depletion layer capacitance at the semiconductor surface which are connected in series. This capacitance depends on the width of the depletion layer and decreases as the width of the depletion layer increases.
In addition, since a positive voltage applied to the gate electrode is increased, minority carriers are finally induced to yield the inversion layer at the semiconductor surface. This state is the inversion. An energy required for the induction of the minority carriers at the semiconductor surface corresponds to at least the band gap energy. However, when the MOS capacitor is shielded from light, an energy obtained from the gate application voltage is too low to induce the minority carriers. Therefore, the inversion layer is not produced during a gate voltage application time, so that the MOS capacitance cannot go into the equilibrium.
On the other hand, the sweep speed of the gate voltage must be reduced in order to obtain a sufficient energy to induce the minority carriers at the semiconductor surface. For example, a measuring time required up to the equilibrium is at most 1 hr per measurement point, so that the measurement efficiency is greatly reduced. Thus, when the MOS capacitor is shielded from light or exposed to light from incandescent or fluorescent lamp, it is impossible to measure an exact MOS capacitance in inversion since the actual MOS capacitance is not in accordance with the theoretical curve. Consequently, it is difficult to obtain exact physical characteristic values, such as the carrier concentration etc., of the MOS device.